Place the chicken breasts between sheets of plastic wrap or parchment paper. Use a meat mallet or rolling pin to gently pound them to an even thickness, about 1/2 inch thick. Season both sides with salt and black pepper.
Set up a breading station with three shallow dishes. Place the flour in one, the beaten eggs in another, and the panko breadcrumbs in the third.
Heat vegetable oil in a large skillet or frying pan over medium-high heat until it reaches 350 F (180 C).
Dredge each chicken breast in the flour, shaking off any excess. Dip it into the beaten eggs, allowing any excess to drip off. Coat the chicken thoroughly with panko breadcrumbs, pressing gently to adhere.
Carefully place the breaded chicken in the hot oil. Fry for about 3-4 minutes on each side or until they are golden brown and cooked through (the internal temperature should reach 165 F or 74 C).
While the chicken is frying, prepare the Tonkatsu sauce. In a small saucepan, combine ketchup, Worcestershire sauce, soy sauce, and sugar. Heat over low heat, stirring until the sugar is dissolved. Simmer for a few minutes until the sauce thickens slightly.
Once the chicken is crispy and cooked through, remove it from the oil and drain on paper towels.
Slice the chicken into strips or serve it whole. Drizzle the Tonkatsu sauce over the chicken, or serve it on the side for dipping.
Serve the Chicken Katsu hot, garnished with sliced green onions or a sprinkle of sesame seeds, if desired.
